This invention relates to the field of electrostatic clamps. In particular, this invention relates to the field of electrostatic chucks for securing semiconductor wafers.
Over the years, designers have developed electrostatic chucks that use ceramics in combination with an embedded metallic conductor (xe2x80x9cceramic chucksxe2x80x9d). The ceramic chuck holds an electrostatic charge that secures a substrate, such as a wafer in a semiconductor manufacturing chamber. These designs typically rely upon an adhesive-type bonding of the assembly to secure the metallic conductor within multiple ceramic layers and to form the ceramic chuck.
Logan et al., in U.S. Pat. No. 5,191,506, describe a ceramic electrostatic chuck that contains multilayer ceramic components supported on a metallic base. The disadvantage of this design is that the electrostatic chuck requires extended charging times to form an adequate electrostatic charge. Typically, this design requires at least about five seconds to store an adequate electrostatic charge. Similarly, this design typically requires at least about ten seconds to discharge the chuck and release the wafer. With the high costs often associated with semiconductor fabrication plants, decreasing the duration of wafer clamping and declamping cycles can measurably increase semiconductor manufacturers"" equipment operating efficiency.
Logan et al., in U.S. Pat. No. 6,268,994, disclose a ceramic electrostatic chuck that relies upon a metal base. Unfortunately, the metal base allows electrostatic bridging between the two poles. This bridging within the metal base results in leakage between the electrodes and the base that can drain-off the stored charge that clamps the workpiece. In addition, this ceramic chuck also requires the extended charging and discharging cycle times experienced with other ceramic chucks.
Before ceramic chucks, most electrostatic chucks relied upon anodized aluminum as the insulator. Logan et al., in U.S. Pat. No. 5,055,964, describe anodized aluminum incorporated into an aluminum base to form an electrostatic chuck. This design forms a strong and effective clamp for most applications. But this chuck""s anodized layer is prone to adsorbing water; and this adsorbed water holds an electrostatic charge. Unfortunately, it is difficult to discharge a polarized water-containing anodized layer; and this results in even greater times to discharge the chuck and release the workpiece.
Since ceramic chucks often have porosity that adsorbs water from the air, they often experience the same discharge problem as anodized chucks. The adsorbed water polarizes electrically in the chuck""s applied electric field. This may occur slowly over long clamping times; and the polarized ceramic chuck does not de-polarize quickly during shorting of the electrodes to release the clamp. By this water-polarization mechanism, the electrostatic chuck can clamp a workpiece long after shorting the chuck""s electrodes. If the electrostatic chuck operated only in a vacuum, then periodically drying out or xe2x80x9coutgassingxe2x80x9d the electrostatic chuck prior to use would solve the retained charge problem. But since some of these chucks operate in an air atmosphere, this is not a practical solution for many such applications where chucks"" performance often degrade over time as they adsorb increasing amounts of water.
The invention is a hybrid chuck for securing workpieces with an electrostatic charge. The hybrid chuck includes a dielectric base for supporting the hybrid chuck. The dielectric base has a top surface and a conductive layer covers at least a portion of the top surface of the dielectric base. The conductive layer is conductive for receiving a current that creates an electrostatic charge and is non-metallic for maintaining the electrostatic charge without significant eddy current losses in the presence of dynamic electromagnetic fields. The top working surface covers the conductive layer and is flat for holding workpieces upon the receiving of the current to create the electrostatic charge in the conductive layer.